Heat-sensitive recording materials

ABSTRACT

A reproduction is obtained by applying a heat pattern to a recording material containing a generally non-light sensitive silver compound which is susceptible to chemical reduction to release silver metal therefrom, while the silver compound is in reactive contact with a 1,2-dihydro-2,2,4-trialkylquinoline reducing agent. The heat effects the reduction reaction in the heated areas releasing silver metal to give a visible indication in the heated areas.

United States Patent Poot et al. 5] Dec. 23, 1975 HEAT-SENSITIVE RECORDING MATERIALS [56] References Cited [75] Inventors: Albert Lucien Poot; Frans Clement UNITED STATES ENTS Heugebaert, both of Kontich; 3,074,809 1/1963 Owen 117/368 Wilhelmus Janssens, Aarschot; 3,094,417 6/1963 Workman 117/362 X Antoon Leon Vandenberghe, Ho 3,446,648 5/1969 Workman l17/36.8 an f Belgium 3,666,477 5/1972 Goffe 96/1 14.1 3,667,959 6/1972 Bojara et al. 96/1 14.1 [73] Assignee: Agfa-Gevaert N.V., Mortsel, 3,679,426 7/1972 Youngquist 117/36.8 X Belgium 3,707,377 12/1972 Tiers et a1 117/368 X [22] Filed: 1973 Primary Examiner-Thomas J. Herbert, Jr. [21] Appl. No.: 335,670 Attorney, Agent, or Firm-William J. Daniel [44] Published under the Trial Voluntary Protest Program on January 28, 1975 as document no. [57] APSTRACT B 33 5 70 A reproducnon is obtained by applying a heat pattern to a recording material containing a generally non- 30 Foreign Application priority Dam light sensitive silver compound which is susceptible to F b 28 1972 d d chemical reduction to release silver metal therefrom, e mte mg om 9141/72 while the silver compound is in reactive contact with a l,2-dihydro-2,2,4-trialkylquinoline reducing agent. [52] US. Cl. 428/457, 96/114.1, 427/145, The heat effects the reduction reaction in the heated 427/148, 427/150, 427/261, 428/488 [51] Int Cl 2 M11 5/18 areas releasing s1lver metal to give a visible indicatlon 58 Field Of Search 117/362, 36.7, 36.8; m the heated areas" 15 Claims, N0 Drawings HEAT-SENSITIVE RECORDING MATERIALS This invention is concerned with heat-sensitive materials suited for the recording and/or reproduction of information and to recording processes wherein such materials are used.

The recording system of the present invention is based on the use of an organic compound that is capable of reducing a noble metal salt as a fatty acid silver salt by the action of heat.

Heat-sensitive copy-sheets capable of undergoing a colour change by the thermally initiated reduction of a silver compound with an organic reducing compound are known from the United Kingdom Patent Specification No. 866,076. Image-forming reactants disclosed in said specification are silver behenate and 2,3-dihydroxybenzoic acid.

A heat-sensitive copy-sheet containing said compounds is suited for the thermographic copying of originals containing infrared-absorbing image markings. When the original is exposed to infrared-radiation in contact with such copy-sheet, the infrared-absorbing image portions of the original are selectively heated and by heat transfer cause the development in the adjacent heat-sensitive sheet of a silver image corresponding with the image markings of the original.

A photo-thermographic process is described in the United Kingdom Patent Specification No. 1,110,046. In said process a photosensitive and heat-developable recording material is applied, which contains:

1. a substantially non-light-sensitive silver salt of an organic acid,

2. a reducing agent, and

3. a catalytic amount of photolytically reducible silver halide being formed from the silver salt and being in catalytic proximity of the silver salt.

Said process comprises the exposure to a light-image of said recording material and the heating of the exposed material in order to cause reduction of the silver salt and image-formation at the light-exposed areas.

According to still another recording and reproduction process an organic reducing compound contained in a photosensitive copy sheet is first rendered inactive and non-transferable by heat through an informationwise exposure in the presence of a compound that helps in the deactivation of the reducing compound. The reducing compound left in the unexposed or less exposed portions is transferred by heat to a receptor sheet on which it causes color formation.

This latter system has the advantage that the final copy is not heat-sensitive and cannot obtain a background coloration anymore, as the reactivity or transferability of the reducing compound has been destroyed information-wise by the photo-exposure.

A two-sheet system in which a photosensitive reductor susceptible of being deactivated by exposure to short wavelength radiation is carried on a sheet separate from the sheet that carries a reducible silver salt has been described in the U.S. Pat. No. 3,094,417 and the United Kingdom Patent application No. 16,999/70.

The present invention provides a process for recording information in which an organic reducing compound under the influence of heat is allowed to effect an information-wise reduction of a reaction partner in order to form an information-wise detectable change.

The organic reducing agent involved corresponds to the following general formula:

wherein:

R represents a C -C alkyl group, for example methyl, ethyl, propyl or butyl, preferably methyl, and

Z represents the necessary atoms to close an adjacent aromatic nucleus, e.g., a benzene nucleus or an adjacent aromatic ring system including a substituted aromatic nucleus or aromatic ring system. Suitable substituents are, e.g., an alkyl group including a substituted alkyl group for example methyl, a halogen atom (F, Cl, Br or I), an alkoxy group, e.g., methoxy, a substituted alkoxy group, e.g., a hydroxy-substituted ethoxy group, an amino group, a substituted amino group, e.g., a monoalkylamino or dialkylamino group or cyano.

The adjacent aromatic ring system closed by the atoms represented by Z is preferably one of the following unsubstituted or substituted nuclei represented by their structural formula:

wherein: R represents hydrogen or an alkyl group,

e.g., methyl or ethyl.

wherein: R represents hydrogen or an alkyl group,

e.g., methyl or ethyl.

i g H (indole) O 3 (dibenzofm-an) i (dibenzothiophene) wherein: X represents oxygen or sulphur x (morass) a (benzothiophene) wherein: X represents oxygen or sulphur Table Continued 3 y p X i g (phenothiazine) V L (phenoxazine) N Formula l V R 4.- (2 wherein: X represents oxygen or sulphur, and 1{1 5 I I R represents hydrogen or an alkyl group, e.g., methyl or ethyl. l

' H I CH5 n H2 5 C HOCH -CH 0- 5 u (indene) 2 v H H H c N (xgnthene) CH H C- 5 O I? i H5 "1 N O= (quinolin-Z-one) 7 g wherein: Ris hydrogen or an alkyl group, e.g.,methyl H or ethyl. 5 Specific examples of the present reducing compounds that are suited for use in a thermally initiated reduction reaction are listed in the following Table. i l H 40 i N CH I'able fcH N0. v Formula v 5 02 l N on 5 5 F 5 Q QEJ 5 I 0H 2. f v H I N I CH I] 5 11 E2 5 H i g H \/N /CH5 CH5 l l 5 5 3,928,686 6 The preparation of the l,2-dihydro-2,2,4-trialkyl- The obtained reaction mixture was subjected to requinolines used according to the present invention duced pressure in order to further remove the volatile proceeds advantageously by condensing a prim ry products. The residue was dissolved in methylene chloamino compound, in which the primary amino group is 5 id d h d i subsequent order: present on an aromatic nucleus, with an aliphatic ke- 3 times with 5N aqueous hydrochloric acid tone containing at least one methyl group linked di- 3 times with aqueous 30 Sodium hydroxide Sohb rectly to the carbonyl group of the ketone in a pretion ferred molar ratio of at least 1:2 in the presence of a 3 times with t suitable catalyst, such as toluenesulfonic acid, benzene- W3 sulfonic acid, sulfuric acid, iodine or bromine. The methylene Chlonde Solutwn was n on y- Examples of suitable ketones are acetone, methyl drous sodfum sulphate and the splvent duvet} ethyl ketone, methyl iso ro l k t d th l b l The residue was purified by boiling it three times with ketone. active carbon and a small amount of a petroleum frac- The production of said organic reducing agents is tion boiling between 80-120C.

illustrated by the following reaction scheme: Yield: 4 g. Melting point: below 50C.

in which Z and R have the same significance as de- 2 PREPARATION OF COMPOUND 4 scribed above.

The following preparations illustrate in more details A mlxture' of mole of bethym'phenymth f t f d aminoquinolin-2-one, 0.65 g of iodine and 100 ml (0.8 blei ac we 0 compoun S enumerated m the Ta mole) of diacetone alcohol was heated for 72 h on an oil-bath. The acetone produced in the reaction was PREPARATION OF COMPOUND 1 separated through a dephlegmator. After the second In a mixture f 0 g f 2 a i fl and 0.6 g f day another 100 ml of diacetone alcohol were added iodine heated at 170C acetone was passed dropwise through the inlet P of a pp tunnel ending for 6 h. The obtained product was dried under reduced below the surface of the reaction q pressure to remove a ton l ft, The reaction product was purified as described for Yield: 98 Melting point: 171C. compound 3.

Yield: 5 g of oily product. PREPARATION OF COMPOUND 2 A mixture of 0.25 mole of 2-amino-diphenylene ox- PREPARATION OF COMPOUND 5 ide, l g of iodine and 2 mole of mesityl oxide was 171 g of p-(B-hydroxyethoxy)aniline and 3.3 g of heated for 12 h on an oil-bath of 140C. The acetone iodine were mixed with 100 ml of acetone. The mixture produced in the reaction was separated thro h a dewas heated on an oil-bath to 170C and 2 liters of acephlegmator. tone were passed in dropwise for 8 h. At the end of that Th b i d reaction mixture was subjected to period the reaction mass was distilled under reduced minished pressure to remove the volatile products. The presosure and the fraction boiling between 145 a residue was dissolved in methylene chloride and 158 at about, mm Hg was collected The fracilon washed in subsequent order was dissolved in methylene chloride and washed in a 3 times with 5N aqueous hydrochloric acid separation funnel with 1N hydrochloric acid. The

methylene chloride part was separated and dried with 3 32: wlth aqueous 30 Sodmm hydmxlde solu' anhydrous sodium sulphate and the methylene chloride evaporated.

3 times with water. Yield: 76

The methylene chloride solution was dried on anhydrous sodium sulphate and the solvent driven off. PREPARATION OF COMPOUND 6 The residue was purified by column chromatography 5 by the use of a column provided with silica gel as adsorber and a mixture of methylene chloride/benzene (8.2 by volume) as eluent. This purification was repeated twice.

In a reaction flask fitted with a feed tube for passing in acetone and a trap for collecting by-product water of condensation, 214 g of p-toluidine (2 mole) and 6 g of iodine were mixed with 100 ml of acetone. The mixture was heated on an oil-bath to 170C and 2 liters of ace- 5 g of Olly product tone were passed in dropwise for 8 h. At the end of that PREPARATION F COMPOUND 3 period the reaction mass was distilled under reduced pressure and the fraction with boiling point 84C at A mixture of 0.1 mole of l-ethyl-4-ammo-carbazole, about 02 mm Hg was collected 0.65 g of iodine and I00 ml of diacetone alcohol was Yield; 97

refluxed on an oil-bath for 3 days and meanwhile diacetone alcohol was added dropwise below the surface of PREPARATION OF COMPOUND 7 the liquid mass. Simultaneously the produced acetone 242 g of 3,4-dimethylaniline (2 mole) and 6 g of was separated by means of a dephlegmator. iodine were mixed with 100 ml of acetone. The mixture was heated on an oil-bath to 170C and 2 liters of acetone were passed in dropwise for 8 h.

At the end of that period the reaction mass was distilled under reduced pressure and the fraction with boiling point 105C at about 0.5 mm Hg was collected.

PREPARATION OF COMPOUND 8 286 g of a-naphthylamine and 6 g of iodine were mixed with 100 ml of acetone. The mixture was heated on an oil-bath to 170C and 2 liters of acetone were passed in dropwise for 8 h. At the end of that period the reaction mass was distilled under reduced pressure and the fraction boiling at 145C at about 1 mm Hg was collected. Said fraction was dissolved in methylene chloride, intimately mixed with 1N hydrochloric acid for 1 hour and subsequently washed with water. The methylene chloride part was separated and dried on anhydrous sodium sulphate. Thereupon the methylene chloride was evaporated and the organic residue dissolved in acetonitrile and ethanol saturated with hydrogen chloride. The mixture was cooled and the precipitate formed sucked off. The separated precipitate was then dissolved in a mixture of water/ammonia (molar ratio /1) and methylene chloride in a same volume added. The methylene chloride layer was separated, dried on anhydrous sodium sulphate and the methylene chloride evaporated.

Yield: 140 g.

According to a preferred embodiment the organic reducing compound is used in a layer or sheet in conjunction with a reaction partner that on heating produces a visible colour change by reduction. In that respect are mentioned, e.g., noble metal compounds that through an oxidation-reduction reaction are capable of separating free metal. Preferred noble metal compounds are silver compounds, which under the conditions encountered in thermographic copying, i.e., preferably in a temperature range of 50 to 200C are poorly light-sensitive, e.g., the silver salts of aliphatic carboxylic acids with a thioether group such as described e.g. in the United Kingdom Patent Specification No. 1,111,492 filed Aug. 13, 1965 by Agfa AG, or silver salts of long chained aliphatic (at least C carboxylic acids such as silver behenate, silver palmitate, silver stearate and others.

Silver behenate and silver stearate are preferred salts. The use of silver behenate or silver stearate in combination with free behenic acid and free stearic acid respectively increases the moisture-resistant properties of the recording composition.

The effectiveness of the reduction obtained with the organic reducing agents of the above general formula varies with the amount thereof. Useful results are obtained already with 0.25 mole of reducing agent per 1 0 ll -O 8 in which:

R represents a saturated or unsaturated aliphatic group or an alkoxy group, preferably having from 1 to 6 carbon atoms, a cycloalkyl group or cycloalkoxy group, e.g., cyclopentyl or cyclohexyl.

These compounds can be prepared according to processes known to those skilled in the art as described, e.g., in Beilstein, Vol. 27, pages 458, and 512, Vol. 27, 2, Erg. page 382, Vol. 21, page 607, Vol. 2, Erg. pages 444 and 445, in Chemical Abstracts, Vol. 54, page 8710 a-c (1960) and in the published German Patent application No. 1,091,976.

As auxiliary reducing agents sterically hindered phenols that on heating become reactive partners in the reduction reaction, e.g., 2,4,6-tri-tert.butylphenol and- /or 2,6-dicyclohexyl-p-cresol may be used.

The reducing agents described in the present invention may be advantageously used in conjunction with a substantially non-light-sensitive silver salt of an organic acid from which a catalytic amount of photolytically reducible silver halide has been formed as described in the United Kingdom Patent Specification No. 1,1 10,047. They may be likewise used in the recording materials containing a higher aliphatic acid silver salt and spectral sensitizing agent described in the German Patent application No. P 2140462.

In the recording materials of the present invention the reactants are preferably applied to a thin flexible carrier or backing such as paper, e.g., glassine paper or baryta-coated paper, or transparent film, e.g., consisting of a cellulose ester or polyethylene terephthalate, in admixture with a film-forming polymeric or resinous binder. The binder can yield a self-supporting sheet or tape when it has a sufficient mechanical strength.

The reactants may be supported in a fibrous web in the absence of a binder. Pigments, e.g., zinc oxide or titanium dioxide, fillers, meltable substances, e.g., waxes, dyes and various other additives may be included for obtaining special effects aimed at.

The reducible compound such as the noble metal salt and the reducing agent are preferably intermixed prior to coating, but may be applied in separate but contiguous layers if desired from which the reducing agent can diffuse to the noble metal salt, e.g., during the heating of the recording material.

As binder agent for the reducing compound and the compound thermally reducible therewith all kinds of natural, modified natural or synthetic resins can be used, e.g., proteins such as gelatin, cellulose derivatives, e.g., a cellulose ether such as ethylcellulose, cellulose esters, carboxymethylcellulose, alginic acid and derivatives, starch ethers, galactomannan, polyvinyl alcohol, poly-N-vinylpyrrolidone, polymers derived from a,B-ethylenically unsaturated compounds, e.g., vinylhomoand copolymers such as polyvinyl chloride, copolymers of vinyl chloride and vinyl acetate, partially saponified polyvinyl acetate, copolymers of acrylonitrile and acrylamide, polyacrylic acid esters, polymethacrylic acid esters or polyethylene.

Heat-sensitive copying materials of the present invention may be used in any process in which heat is information-wise applied to or generated in the copying material, e.g., by means of hot bodies for example a hot stylus or by means of heat producing radiation.

Heat-sensitive sheet-like copying materials of the present invention are particularly suited for the thermographic copying of originals containing infraredabsorbing image markings. When the original in contact with the present copying material is exposed to infrared radiation, the infrared-absorbing image portions of the original are selectively heated and by heat transfer cause the formation in the adjacent heat-sensitive sheet of a visible color-change resulting from the reaction of the organic reducing agent with a suitable reaction partner of an oxidation-reduction reaction, e.g., a noble metal salt.

According to another exposure technique the copying material is heated information-wise or image-wise by radiation absorbed in the copying material. In order to improve the absorption of copying radiation, which may be light and/or infra-red radiation, the copying material in heatconductive relationship with the oxidation-reduction partner(s) contains a certain amount of a substance or substances that are capable of converting absorbed electromagnetic radiation energy into heat. Suitable substances for that purpose are, e. g., heavy metal particles and finely divided carbon. For more details about such exposure technique and the use of these substances reference is made to the U.S. Pat. No. 3,476,937 and the United Kingdom Patent Specification No. 1,160,221.

Heat-sensitive copying materials of the present invention containing said noble metal salts are suitable 'for making direct, high contrast, clear detail, black permanent copies of typewritten, printed and other graphic subject matter. The copying materials are stable at room and normal storage temperatures, and may be stored in daylight without visible change either before or after heat-copying when no light-sensitive silver halide is present in their composition.

The following examples illustrate the present invention. The percentages are by weight unless otherwise indicated.

EXAMPLE 1 A mixture containing the following ingredients was ball-milled for 12 hours:

ethyl acet ate stearic acid "polymethyl" hethacrylate (the viscosity of a 5 76 solution in p l rqipm at. 2 is -9 .SPf-EE cellulose acetate butyrate (substitution degree of acetate and butyrate 0.5 and 2.4 respectively the viscosity of a 20 solution in zinc oxide (average grain size ais q slsilver stearate mm m Before coating the following ingredients were added to the ball-milled composition:

ethyl acetate compound 5 of Table l EXAMPLES 2-3 Example 1 was repeated with the compounds 6 and 7 of Table I and the same results as described in Example 1 were obtained with the thus modified recording materials.

We claim:

1. A recording process comprising the step of applying a heat pattern corresponding to an image to a recording material containing a silver compound that is generally non-light sensitive at a temperature below about 200C and is adapted to undergo chemical reduction to release silver metal, while said silver compound is in reactive contact with an organic reducing compound corresponding to the following general formula:

-H n on R 3 Z I in which:

R represents a C -C, alkyl group, and

Z represents the necessary atoms to close a benzene, naphthalene, anthracene, carbazole, indole, benzo furan, dibenzofuran, phenothiazine, tluorene, indene, xanthene, or quinolin-2-one nucleus,

whereby said silver compound is reduced in the heated regions of said material to silver metal and forms a visible image of said pattern.

2. A recording process according to claim 1 wherein said recording material is a recording layer and both said reducing compound and said silver compound are present in the same recording layer.

3. A recording process according to claim 2 wherein said heat pattern is formed by exposing said recording material to infrared while said layer is in heat-conductive contact with an original to be reproduced carrying infrared-absorbing image-markings.

4. A recording process according to claim 1 wherein said heat pattern is formed by contacting said recording material with a hot stylus according to said image.

5. A recording process according to claim 1 wherein said heat pattern is formed by exposing said material to imagewise modulated electromagnetic radiation, which is absorbed in said material and converted therein into heat.

H r 3 Z A in which:

R represents a C -C alkyl group, and Z represents the necessary atoms to close a benzene, naphthalene, anthracene, carbazole, indole, benzo furan, dibenzofuran, phenothiazine, fluorene, indene, xanthene, or quinolin-2 -one nucleus,

and having in said layer or in a contacting layer a silver compound reducible by said organic reducing compound to silver metal under the influence of heat, said silver compound being generally non-light sensitive at a temperature below about 200C.

10. A sheet material according to claim 9 wherein said recording layer also contains a toning agent.

11. A sheet material according to claim 9, wherein the silver compound is a silver salt of an aliphatic carboxylic acid containing a thioether group.

12. A sheet material according to claim 9, wherein the silver compound is a silver salt of an aliphatic carboxylic acid containing at least 14 carbon atoms.

13. A sheet material according to claim 12, wherein the silver compound is silver behenate or silver stearate.

14. A sheet material according to claim 9, wherein the reducing compound is present in said layer in an amount of at least 0.25 mole per mole of said silver compound.

15. A sheet material according to claim 9, wherein the organic reducing compound and said reducible silver compound are used in admixture with a binder applied in layer form on a support.

g i I I i L.

UNITED STATES PATENT AND TRADEMARK OFFICE CERTIFICATE OF CORRECTION PATENT NO. 3 ,928 ,686 DATED December 23, 1975 |NyENT0R(5) Albert Lucien Poot et al it is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

In the Claims: Column 10, line 58 (Claim 3), after "infrared" insert radiation Signed and Scaled this A ttest:

RUTH C. MASON C. MAR Arrestin HALL DANN vmmrssmner uflarenls and Trademarks 

1. A REDORDING PROCESS COMPRISING THE STEP OF APPLYING A HEAT PATTERN CORRESPONDING TO AN IMAGE TO A RECORDING MATERIAL CONTAINING A SILVER COMPOUND THAT IS GENERALLY NON-LIGHT SENSITIVE AT A TEMPERATURE BELOW ABOUT 200*C AND IS ADAPTED TO UNDERGO CHEMICAL REDUCTION TO RELEASE SILVER METAL, WHILE SAID SILVER COMPOUND IS IN REACTIVE CONTACT WITH AN ORGANIC REDUCING COMPOUND CORRESPONDING TO THE FOLLOWING GENERAL FORMULA: 2-(CH3-),2,4-DI(R-),5,6-(-Z-)-1,2-DIHYDROPYRIDINE IN WHICH: R REPRESENTS A C1-C4 ALKYL GROUP, AND Z REPRESENTS THE NECESSARY ATOMS TO CLOSE A BENZENE, NAPHTHALENE, ANTHRACENE, CARBAZOLE, INDOLE, BENZO FURAN, DIBENZOFURAN, PHENOTHIAZINE, FLUORENE, INDENE, XANTHENE, OR QUINOLIN-2-ONE NUCLEUS, WHEREBY SAID SILVER COMPOUND IS REDUCED IN THE HEATED REGIONS OF SAID MATERIAL TO SILVER METAL AND FORMS A VISIBLE IMAGE OF SAID PATTERN.
 2. A recording process according to claim 1 wherein said recording material is a recording layer and both said reducing compound and said silver compound are present in the same recording layer.
 3. A recording process according to claim 2 wherein said heat pattern is formed by exposing said recording material to infrared while said layer is in heat-conductive contact with an original to be reproduced carrying infrared-absorbing image-markings.
 4. A recording process according to claim 1 wherein said heat pattern is formed by contacting said recording material with a hot stylus according to said image.
 5. A recording process according to claim 1 wherein said heat pattern is formed by exposing said material to imagewise modulated electromagnetic radiation, which is absorbed in said material and converted therein into heat.
 6. A recording process according to claim 1, wherein the silver compound is a silver salt of an aliphatic carboxylic acid containing at least 14 carbon atoms.
 7. A recording process according to claim 6, wherein the silver salt is silver behenate or silver stearate.
 8. A recording process according to claim 1, wherein the organic reducing agent is used in an amount of at least 0.25 mole per mole of said silver compound.
 9. A SHEET MATERIAL COMPRISING AT LEAST ONE RECORDING LAYER CONTAINING AN ORGANIC REDUCING COMPOUND CORRESPONDING TO THE FHOLLOWING GENERAL FORMULA: 2-(CH3-),2,4-DI(R-),5,6-(-Z-)-1,2-DIHYDROPYRIDINE IN WHICH: R REPRESENTS A C1-C4 ALKYL GROUP, AND Z REPRESENTS THE NECESSARY ATOMS TO CLOSE A BENZENE, NAPHTHALENE, ANTHRACENE, CARBAZOLE, INDOLE, BENZO, FURAN, DIBENZOFURAN, PHENOTHIAZINE, FLUORENE, INDENE, XANTHENE, OR QUINOLIN-2-ONE NUCLEUS, AND HAVING IN SAID LAYER OR IN A CONTACTING LAYER A SILVER COMPOUND REDUCIBLE BY SAID ORGANIC REDUCING COMPOUND TO SILVER METAL UNDER THE INFLUENCE OF HEAT, SILVER COMPOUND BEING GENERALLY NON-LIGHT SENSITIVE AT A TEMPERATURE BELOW ABOUT 200*C.
 10. A sheet material according to claim 9 wherein said recording layer also contains a toning agent.
 11. A sheet material according to claim 9, wherein the silver compound is a silver salt of an aliphatic carboxylic acid containing a thioether group.
 12. A sheet material according to claim 9, wherein the silver compound is a silver salt of an aliphatic carboxylic acid containing at least 14 carbon atoms.
 13. A sheet material according to claim 12, wherein the silver compound is silver behenate or silver stearate.
 14. A sheet material according to claim 9, wherein the reducing compound is present in said layer in an amount of at least 0.25 mole per mole of said silver compound.
 15. A sheet material according to claim 9, wherein the organic reducing compound and said reducible silver compound are used in admixture with a binder applied in layer form on a support. 